1. Field of the Invention
The present invention relates to a focus detection method for detecting the focus state of a digital camera on an object in accordance with the contrast of an image thereof captured by the digital camera. The present invention also relates to a focus detection apparatus using such a focus detection method.
2. Description of the Related Art
A contrast detection method is known as a conventional focus detection method for compact AF digital cameras. Conventional focus detection apparatuses using the contrast detection method operate to determine the position of a focusing lens group in which contrast of an object image captured by an image pickup device becomes maximum. Therefore, in such conventional focus detection apparatuses, an imaging operation is performed successively at different positions of the focusing lens group while the focusing lens group is driven stepwise from the infinite focus position (position for bringing an object at infinity into focus) to the closest (shortest) focus position to obtain a contrast from a captured image signal at each different position of the focusing lens group, and a maximum value (peak value) of contrast among the contrasts obtained at the aforementioned different positions of the focusing lens group is determined so that the focusing lens group is driven to a position thereof where the maximum value of contrast is obtained to bring the object into focus. Such a focus detection apparatus is disclosed in, e.g., Japanese Unexamined Patent Publication No. 2001-249267.
In the conventional contrast detection method, the difference between two contrasts obtained at two different positions of the focusing lens group by moving the focusing lens group in the optical axis direction is small in the case of a low-contrast object image. Nevertheless, in the conventional contrast detection method, a reversal point where either the contrast (contrast value) of a captured image commences to decrease after increasing more than once (see FIG. 10A) or the contrast of a captured image commences to decrease more than once after increasing (see FIGS. 10B and 10C) is detected as a peak value (maximum value) of contrast, or only one of the number of successive increments of the number of successive decrements is counted to detect a peak value (maximum value) of contrast. Therefore, if the peak value (maximum value) of contrast is detected in each of a low-contrast object image and a normal (high-contrast) object image using a common criterion of judgment, a peak value of contrast cannot be detected in the low-contrast object image. Additionally, there is a possibility of the peak value of contrast in a low-contrast object image being determined as not being credible if the peak value (maximum value) of contrast is detected in each of a low-contrast object image and a normal object image using common criterion because the credibility of the detected peak value of contrast is determined from the magnitude of the contrast in the conventional contrast detection method.